Just wanted to set the record straight here about the Cummins DEF system. DEF doesn't have any effect on the DPF because DEF is injected post-DPF, and DEF has nothing to do with particulate reduction. It only helps to convert harmful gases into harmless nitrogen (N2) and water (H2O). The DPF still catches particles, and the system will still go into regen. The manual says that you need to drive at freeway speeds for 45 minutes to clear out the filter.
Here's a video on how the Cummins SCR system works. It's a 2010 video, because this system was first implemented on 2011 model year Ram chassis cab trucks.
2010 Cummins Aftertreatment System Animation - YouTube
If you'd like to learn more about DEF itself and the chemical process that happens when DEF is injected into the exhaust, check out this PDF:
http://www.cumminsfiltration.com/pdfs/product_lit/americas_brochures/MB10033.pdf
I'm about a month away from receiving my new truck, and I've been doing a lot of research on this DEF system over the past 8 months to determine if it was better or worse to go with a 2013 or a 2012. The claim is that the 2013 trucks should get better fuel mileage, but we'll see how that goes. The additional cost of DEF is minimal. It's used at a 50:1 diesel to DEF ratio. This means that for every 50 gallons of diesel fuel used, the system uses roughly one gallon of DEF. If Diesel is $3.60 (today's price at my station around the corner), that means if you were to spend $180 on diesel, you'll now spend $182.79. My closest truck stop with DEF sells it for $2.79/gallon. Overall cost of operation increases by 1.5%. Equivalent to a 4 cent increase in the cost of diesel from $3.60/gallon.
While some like to consider this a "liberal" system, and indeed since it's a "save the earth" hippie thing, that's somewhat true. But if you've ever been to a country where diesel is heavily subsidized, and over half the vehicles are spewing black smoke in your face as they drive by, these are systems that may not make your butt-dyno happy, but they will reduce overall emissions, and give diesels a better image overall, and give us cleaner local air, even if the planet itself is already fūcked. The claim is that with this system, the engine has near-zero emissions. Lower than many of today's gasoline engines. As for repair costs, only the future will tell. I think the ultimate question is whether or not the DEF system is part of the powertrain warranty.
